Java 2 Platform, Enterprise Edition (J2EE) uses a multi-tier distributed model. The J2EE Platform consists of a Web Server and an EJB Server. (These servers are also called "containers.") The Web container provides the runtime environment through components that provide naming context and life cycle management. Some Web servers may also provide additional services such as security and concurrency control. A Web server may work with an EJB server to provide some of those services. A Web server, however, does not need to be located on the same machine as an EJB server. The EJB server provides an environment that supports the execution of applications developed using Enterprise JavaBeans (EJB) components. It manages and coordinates the allocation of resources to the applications. Enterprise beans typically contain the business logic for a J2EE application.
Also, see EJB.
Organization: Java Community Process (JCP)
More information: J2EE page on the Oracle site
More on the general topic: Application Servers